1. Technical Field
The invention relates to the field of interference suppression in telephone networks and, more particularly, to a circuit arrangement for suppressing interference signals on the receiving arm of a subscriber line circuit which is constructed from electronic components and thus avoids the use of speech transformers. More particularly, it relates to connecting a two-wire subscriber line circuit to the exchange of a digital time division multiplex telephone system, where interference signals occur on subscriber lines as a result of change pulse signals intentionally fed-in to the transmitting path and which are intended for the subscriber station connected to the exchange via the subscriber lines.
2. Description of the Prior Art
The transmission of charge pulses to subscriber stations which are connected via two-wire subscriber lines to an exchange for the purpose of charge metering at the subscriber station is carried out by transmitting alternating voltage pulses at frequencies which are outside the speech band and which amount, for example, to 12 kHz or 16 kHz.
Without the provision of special measures, these pulses can return from the subscriber station to the receiving arm of the subscriber line circuit and from there to the analog to digital conversion devices. At the large amplitude necessary for charge metering, this kind of undesired influence on these A/D converter devices can considerably impair their proper functioning.
In conventional exchange systems in which a.c. pulses (16 and 12 kHz) are fed-in symmetrically and in which a speech transformer is provided for the output coupling of the speech signals to be supplied to the coding device, interference signals are suppressed in a known manner by means of a filter Fi, representing a four-pole circuit, in the form of a parallel resonator. However, the outlay and set requirements for such a filter are considerable, and, in addition, the line symmetry of the subscriber line is disturbed resulting in the known outcome of sensitivity to series voltages and a noisy line.